The present invention relates to animated bi-level display systems and, in particular, to a method and arrangement for minimizing scintillations in animated dithered displays.
At the heart of a bi-level display system is a display panel typically comprising a matirx of individual, closely spaced display cells each of which resides in one of two visual states. That is, each display cell is either completely energized (on) or completely de-energized (off). Picture images and other graphic data are readily displayed on a bi-level display panel via selective energization of its cells.
Since the cells of a bi-level display panel are either completely on or off, the panel has no inherent capability for representing gray scale in reproduced images. Advantageously, however, it is known that a subjective impression of gray scale can be produced by way of a technique known as "dither processing." In a so-called "dithered display system" the observer is made to perceive various shades of gray, i.e., variations in intensity, in the reproduced imaage by appropriate arrangement of on and off cells.
Dither is implemented in a bi-level display system by dividing the image to be reproduced into a matrix of picture elements, each element corresponding to a respective cell of the display panel. A predetermined dither threshold value is assigned to each display cell. If the intensity of any given picture element is greater than the dither threshold value assigned to the corresponding display cell, that cell is turned on. Otherwise, it is maintained off.
A bi-level display system can be adapted to present animated dithered images by providing therein circuitry for displaying sucessive dithered frames at a rate of sufficient for the eye to integrate the frames into smooth, continuous motion. A "write", or "energize" signal is extended to each cell which is to be on for a given frame. An "erase", or "de-energize", signal is extended to each cell which is to be off for that frame. However, this animation technique requires that the display cells be able to be accessed to receive a write or an erase signal at a very rapid rate since each cell must be accessed in each frame. Unfortunately, the cells in some bi-level display panels (including, for example, most commercially available plasma display panels) cannot be accessed fast enough to display a sufficiently large number of frames per second to present a pleasing animated image.
Alternatively, as disclosed in the copending U.S. patent application of C. N. Judice, Ser. No. 546,863 filed on the same day as this application and assigned to the same assignee now U.S. Pat. No. 3,937,878, issued Feb. 10, 1976, animated images can be presented advantageously in such "slow" dithered display systems by utilizing therein a technique known as "conditional replenishment." In accordance with this technique, the only display cells which are accessed to receive a "write" or an "erase" signal in any given frame are cells which are to have states in that frame which differ from their respective states in the previous frame. The remaining cells are not accessed at all. Rather, they are maintained in their respective previous on or off states.